Anyway, great conversation. I do hope that non-grimdark SF becomes a think, because Crapsack worlds are becoming tired to me.

A question you all didn’t address but I am curious–when did the switch happen? SF had a publishing dominance over fantasy for a while. Did it start with Terry Brooks and Tolkien clones? Can we blame all this on the 70’s?

That’s definitely part of it. The other big factors, IMO, were the change in the structure of the publishing market — basically the collapse of short-fiction magazines — and the increasing implausibility of optimistic SF. By the late 70s it was clear that the future was going to be more Neuromancer than 2001. So I’d put it in the late 70s-early 80s?

I don’t think that the “suspension of disbelief” theory does a good job of explaining how people actually interact with art. But a lot of people have certainly internalized it and made it a dominant part of how they critique art. Education or intelligence do not disable our ability to play along with a story, or to meet art on its own terms. Demanding to be convinced is a conscious choice.
There’s no scientific reason to doubt that people will wear blue jeans in the future. There are many materials, styles, etc. that persist in spite of the passage of huge spans of time, and significant technological change. There’s no Law of Fabrics that says “once X level of technology is reached, denim will no longer be fashionable, durable, or comfortable”. It’s not science or believability that makes people question things like that, it’s simply the current fashion of criticism.
This is quite the opposite of meeting art with a willingness to believe in it, and maybe being knocked out of that belief by an element that doesn’t work. It’s meeting art with the attitude that “I will refuse to believe this art until it proves to me that it has earned my suspension of disbelief.”
This attitude is applied with gusto to science fiction, because it’s assumed that science fiction is always about accurate representation of material facts, and accurate predictions about how technology will be used in the future. It’s easy to check these facts. Either a rocket trip to Mars takes three months or it doesn’t. You can look that up on Wikipedia and confirm whether or not that part of the story deserves your suspension of disbelief, and confirm to yourself that you’re being a smart reader by not believing in an unworthy thing.
This approach doesn’t work as well with fantasy because there’s no conceit that any of it is supposed to track with real-world facts. You can speculate about how it ought to work if it were actually real. Or, you can compare the fantasy world to its real-world inspirations and make judgements about how faithful it is. Or you can judge whether a writer is being consistent with whatever rules they’ve set out for themselves. But you can’t legitimately argue about whether “summon elemental” really works that way.
We don’t doubt the believability of rom-coms because we’re convinced that every happy couple actually, factually meets in a cute way, and goes through a phase of hating each other before they realize that they love each other. Does “Pretty Woman” earn our suspension of disbelief? Of course not. So we don’t ask it to. There’s no cred to be had in picking apart the logic of a genre that is so blatantly escapist. Ditto all genres that are primarily about emotional satisfaction (like revenge flicks, or stories about underdogs overcoming great odds, etc.). But you can impress people (including yourself) by knowing that “Jurassic Park” got stuff wrong about dinosaurs.
If believability is a reason that science fiction is less popular now, it’s because current readers have adopted special prejudices for it that they don’t apply to other genres. Smart, well educated readers are perfectly capable of enjoying SF that doesn’t present every detail “correctly”, until they decide that they aren’t. It’s also possible to enjoy hard SF that does strive for rigorous plausibility, while also enjoying more fanciful versions of SF, unless you tell yourself that you can’t.
If people were less incredulous about the believability of old school “space patrol” sf, it wasn’t because their lack of scientific knowledge allowed them to suspend their disbelief. It was because they hadn’t accepted the idea that they ought to be so parsimonious with their belief. They assumed that you read that stuff because it was entertaining, and fun, and cool. It doesn’t take a rocket scientist to figure out that rocketships don’t really work the way they do in Buck Rogers, but that’s not a problem until you decide that it should be.